Li Zuixiong, Neville Agnew, and Po-Ming Lin
Figure 1
Gravel plateau above the Mogao grottoes, connected to the Mingsha mountain in the west. A windbreak fence was built in front of the mountain.
The mogao grottoes were excavated in a north-south-oriented cliff face on the west bank of the Daquan River. The cliff face is 1,680 m long and approximately 20 m high. At the base of the cliff is a relatively flat sand-and-gravel ground. The cliff is about 90 m at its closest point to the Daquan River and about 180 m at the farthest point from it. Trees have been planted on the flat, elongated area in front of the cliff. The caves are excavated in the south 1,000 m of the cliff face. In the northern area, except for a few caves that are decorated with murals and polychrome statues, most of the caves are undecorated and served as living quarters for craftsmen, monks, and pilgrims. Toward the top of the cliff in the southern region is a 35° erosional slope 30 m in width partly covered in sand and gravel with exposed surfaces of conglomerate. At the top of the slope is a 3,000 x 800 m plateau composed of sand and gravel, leading to the Mingsha mountain in the west (Fig. 1).
This region is characterized by large sand dunes, and winds blow from several directions seasonally. The westerly winds bring sand from the dunes of the Mingsha mountain, causing erosion of the cliff rock and damage to the grotto areas.
On the basis of estimates, 3,000 m3 of sand accumulated annually in front of the cliff face before the installation of the windbreak fence.
The sand blocked walkways and entrance doors, and fine dust infiltrated the grottoes, settling on the statuary and wall paintings. Removal of such a large sand accumulation required considerable annual expenditures of money.
A knitted synthetic fabric windbreak fence in an A shape (as viewed from above) was installed on the top of the cliff to control the windblown sand carried by the westerly winds. The apex of the triangular windbreak fence is about 70 m from the Mingsha mountain and is pointed toward it; its base is about 800 m in length. It is parallel to the cliff face and positioned about 200 m from it. All the southern caves are enclosed in the area protected by it (Fig. 2).
Since its erection, the windbreak fence has effectively prevented sand blowing from the Mingsha mountain from entering the grotto area.
Figure 2
Schematic drawing showing the locations of the A-shaped windbreak fence and chemical consolidation test area.
This has resulted in the sand carried by the wind being deposited around the windbreak fence or diverted to outside the grotto area, thus reducing the sand deposit in the grotto area by 60%. However, since the installation of the windbreak fence, the balance of supply of sand to the cliff face has been disturbed. Wind has stripped the 30° slope and plateau behind it of its residual sand, exposing the weak conglomerate of the cliff to more rapid erosion than it experienced previously (Fig. 3). Analysis of the sand grains collected from sand traps placed on the walkways and around the grotto area shows that after the installation of the windbreak fence, coarser sand grains and even pebbles were deposited (Ling et al. 1993).
Figure 3
Severely eroded strata at the Mogao grottoes.
The upper layers of the Quaternary argillaceous conglomerate cliff rock are loosely cemented and easily eroded by wind, causing pebbles to fall from the top of the cliff. This is not only dangerous to pedestrians on the walkways below, but thinning of the rock of the roofs of some of the upper-level caves—a phenomenon already well advanced—will further endanger these caves. It is presently considered that it will probably be necessary to chemically consolidate the sand in front of the windbreak fence, and the exposed rock of the cliff slope. In fact, the latter is the principal objective of the testing described in the present paper because of the ease of erosion of the exposed rock on the 30° slope. With the installation of the windbreak fence completed, testing of chemical consolidation became the next priority, the use of both the windbreak fence and chemical consolidation being planned as a comprehensive sand-control system.
Experimental Work
World History
